The cornea is responsible for approximately two-thirds of the refractive (light bending) power of the human eye. This is largely due to the high refractive index at the air/tear film interface. The crystalline lens (located between the cornea and the retina) is responsible for the other third of the eye's total refractive power. In effect, each eye contains two lenses that act to bend or focus light coming into the eye: the cornea and the crystalline lens.
Astigmatism is a condition in which the shape of a lens is warped so that it is steeper across one meridian and flatter across another. Modern visualization and mapping techniques can measure the deformation of the cornea. Corneal distortion is caused by one section of the cornea being flatter than another. Such distortion is commonly symmetric. That is, if a fine line were drawn across a cornea where the cornea was flattest, the cornea would be most sharply curved on a line perpendicular to the first line. This warping distorts and blurs an individual's vision. While astigmatism is generally caused by a deformation of the cornea, it can also be present within the crystalline lens.
For decades surgeons have been replacing the natural lenses of patients' eyes with intraocular lenses (IOLs) to deal with astigmatism and other visual maladies. IOLs are usually made of a compound such as silicone or acrylic plastics. When an IOL is implanted, the eye is termed “pseudophakic”.
During refractive cataract surgery a surgeon removes a cataractous lens which is cloudy or opaque and often will attempt to correct preexisting vision problems such as myopia, hyperopia or astigmatism. Ideally after cataract surgery the patient will no longer need contacts or glasses for the operated eye(s).
Surgeons also correct corneal astigmatism and/or cataracts using Toric IOLs which offer both spherical and cylindrical corrections. IOLs are also used to correct other optical refractive errors such as presbyopia. For example, accommodating or multifocal IOLs can be used to restore reading vision. These lenses traditionally require the correction of corneal astigmatism to work best.
Other implants for correcting vision are multifocal IOLs and corneal inlays. Both of these implants are designed to offer better accommodation, usually for close range focus. Corneal inlays are placed in a pocket created in the cornea by a laser and create an increased depth of field for the patient. Near perfect alignment of the implant (which is often needed or desired for the outcome to be beneficial) requires modern techniques.
When astigmatism is corrected using a Toric IOL, the IOL is shaped to counteract the distortion of the cornea. In order for this correction to properly occur, the IOL is turned or rotated to a position, which is aligned with the corneal astigmatism and preferably centered relative to the patient's visual axis.
Modern surgical procedures intending to correct or enhance an individual's visual acuity rely on the overall accuracy the surgeon is able to place with his or her incisions, tissue ablations, implants and/or sutures. In order for an IOL to work properly, it should be accurately aligned with the cornea. If the IOL is improperly aligned with the corneal astigmatic meridians (often only by a few degrees), the patient's vision will likely be suboptimal. Many corneal inlays require axis accuracy with error no greater than 0.3 mm decentered from the visual axis. A misalignment of 5 degrees can cost the patient 15 percent of the desired effect. Therefore, it is important that astigmatic correcting IOLs be properly aligned with the patient's corneal aberrations.
Most IOLs have two small supports called haptics which engage the peripheral capsule (fornices). The haptics hold the IOL in place so that it does not rotate within the eye while at the same time centering the optic within the pupil. Once an IOL has been placed in the eye, the haptics bind to the interior of the eye by fibrosis of the capsular bag making it difficult, if not impossible, to reposition the IOL. Often the only solution for a misaligned IOL is an IOL exchange which can lead to a host of complications.
When implanting an IOL or a similar device, surgeons often place pre-operative marks on the cornea to aid in the alignment and implantation of IOLs as it is often difficult to see whether the IOL is aligned properly with the visual axis of the optic system.
Traditional pre-marking systems rely on the patient fixating on a given object (traditionally the ear of the surgeon or another object behind the surgeon). This method is prone to error as it is extremely difficult, if not impossible, to correctly align the IOL with the patient's visual axis using this technique.
As stated above, proper alignment of these IOLs and similar devices can be important, if not crucial, for optimum, or at least satisfactory, optical results. Toric IOLs, accommodating IOLs, multi-focal IOLs and simple spherical IOLs benefit from proper centration and alignment. Future advances in IOL development such as Higher Order Aberration (HOA) correcting lenses will also depend heavily on near perfect alignment. Current pre-marking methods fail to orient the corneal reference marks with the patient's visual axis (the center line of the optical path) as the origin.